This invention relates to an X-ray tube anode target and, more particularly, to a special coating on a rotating anode target for increased heat emissivity purposes.
Ordinarily an X-ray beam generating device referred to as an X-ray tube comprises dual electrodes of an electrical circuit in an evacuated chamber or tube. One of the electrodes is a thermionic emitter cathode which is positioned in the tube in spaced relationship to a target anode. Upon energization of the electrical circuit, the cathode is electrically heated to generate a stream or beam of electrons directed towards the target anode. The electron stream is appropriately focussed as a thin beam of very high velocity electrons striking the target anode surface. The anode surface ordinarily comprises a predetermined material, for example, a refractory metal so that the kinetic energy of the striking electrons against the target material is converted to electromagnetic waves of very high frequency, i.e. X-rays, which proceed from the target to be collimated and focussed for penetration into an object usually for internal examination purposes, for example, medical diagnostic procedures.
Well known primary refractory metals for the anode target surface area exposed to the impinging electron beam include tungsten (W), molybdenum (Mo), and their many alloys for improved X-ray generation. In addition, the high velocity beam of electrons impinging the target surface generates extremely high and localized temperatures in the target structure accompanied by high internal stresses leading to deterioration and breakdown of the target structure. As a consequence, it has become a practice to utilize a rotating anode target generally comprising a shaft supported disk-like structure, one side or face of which is exposed to the electron beam from the thermionic emitter cathode. By means of target rotation, the impinged region of the target is continuously changing to avoid localized heat concentration and stresses and to better distribute the heating effects through out the structure. Heating remains a major problem in X-ray anode target structures. In a high speed rotating target, heating must be kept within certain proscribed limits to control potentially destructive thermal stresses particularly in composite target structures, as well as to protect low friction high precision bearings which support the target.
A target body is chosen from a material with a high heat storage capacity because most of the heat transfer must take place through radiation from the target to the X-ray tube or envelope structure. For example, only about 1.0% of the energy of the impinging electron beam is converted to X-rays with the remainder appearing as heat which must be rapidly dissipated from the target essentially by means of heat radiation. Accordingly, significant technological efforts are expended towards improving heat dissipation from X-ray anode target surfaces.
One preferred material for a rotating disk-like anode target is graphite (C) which has a high heat storage capacity and which readily accepts bonding of a refractory metal cover or surface as the cathode electron beam impinging surface. It is further imperative that good heat dissipation be provided for the composite structure of a graphite body with a refractory metal surface. Rotation of targets for improved heat dissipation and radiation has progressed to target speeds exceeding 10,000 rpm with elevated temperatures of 1200.degree. C. and above, conditions which exacerbate potential defect sites associated with the metal surface or graphite body.